Exhaust systems typically muffle noise produced by combustion processes within engines. At a minimum, a typical exhaust system usually includes an exhaust pipe to carry engine exhaust gases and sound away from the engine, and a muffler to attenuate the sound propagated through the exhaust pipe. Mufflers include two general types according to the mode by which noise is attenuated. Mufflers that attenuate noise by reflection of sound waves are called reactive or reflection mufflers. Mufflers that attenuate noise by absorption of sound waves are known as dissipative or absorption mufflers.
Reflection mufflers are particularly useful for low-frequency applications and for high-temperature applications that restrict or preclude use of absorption mufflers. Reflection may be provided by resonators or changes in exhaust flow direction by labyrinth-like baffling in the muffler. Reflection mufflers usually include a hollow steel housing defining an expansion chamber and one or more baffles and/or resonator chambers in communication with the expansion chamber, a steel inlet pipe extending into the expansion chamber, and a steel outlet pipe extending from the expansion chamber to the outside. Sound waves enter the main chamber through the inlet pipe, and reflect off various baffles or other surfaces in the chambers to cancel each other out and thereby reduce noise. Reflection mufflers may produce undesirable backpressure.
Current absorption mufflers may be used in applications where low pressure drop and high attenuation at predominantly middle and high frequencies are required. Absorption mufflers typically include a steel housing defining one chamber, a perforated pipe extending completely through the chamber of the housing, and absorption material disposed in the chamber between the pipe and the housing. Sound waves enter the chamber through the perforated pipe, and become absorbed by the absorption material. Until now, absorption mufflers generally produced less sound control than reflective mufflers.